[racket] Determining type from a syntax object

It would be useful in the case of designing macros. The macro could make
decisions based not only on syntax but on type.

Although to be honest (and to answer Robby's earlier question) I came
across this while avoiding a longer but better way to design my program.

--Spencer


On Mon, Jun 16, 2014 at 2:46 PM, Alexander D. Knauth <alexander at knauth.org>
wrote:

>
> On Jun 16, 2014, at 3:41 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu>
> wrote:
>
> > On Mon, Jun 16, 2014 at 3:29 PM, Alexander D. Knauth
> > <alexander at knauth.org> wrote:
> >>
> >> On Jun 16, 2014, at 3:13 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu>
> wrote:
> >>
> >>> On Mon, Jun 16, 2014 at 3:06 PM, Alexander D. Knauth
> >>> <alexander at knauth.org> wrote:
> >>>>
> >>>>> Typed Racket functions are
> >>>>> just plain Racket functions. We could add some extra metadata to
> every
> >>>>> value that held its type,
> >>>>
> >>>> It wouldn’t have to be every value, it could just put that
> information in for :has-type expressions, right?
> >>>> Would that still require fundamental changes to Racket?
> >>>
> >>> The problem is that the information needs to get attached to the value
> >>> where the value is created, not where you use `:has-type?`.  The
> >>> `:has-type?` could even be in some other module, so you'd have to add
> >>> this information to every value, everywhere.  That's why it would need
> >>> to be part of the runtime.
> >>
> >> Then how would ann do it?
> >>
> >> I was thinking of something that would work sort of like ann, except
> that if it has the type, it would put true there (instead of the value), and
> >> if it doesn’t, then it would put false there (instead of raising an
> error).
> >>
> >> I understand that there’s no way to do this at ‘expansion’ time because
> it type checks it after it expands, but is there any way
> >> to do this at ‘type-check’ time?  That’s what ann does, right?
> >>
> >> That way the actual checking wouldn’t have to happen at runtime.
> >>
> >> Or do I have this expansion-time, type-check-time, run-time thing wrong
> somewhere?
> >
> > I think you have the various times right, but this proposal won't work
> > either. In particular, now your original example won't work:
> >
> > (cond [(:has-type? f (Number -> Any))
> >       (f 1)]
> >      [(:has-type? f (String -> Any))
> >       (f "1")])
> >
> > Imagine the `f` has type `Any at the start.  Then if you replace
> > `:has-type?` with `ann`, you get a type error.
> >
> > Unless you're suggesting that `f` would have to already have one of
> > those types, in which case I'm not sure how it helps you.
> >
> > Sam
>
> Oh, I hadn’t realized that.  f would have to already have one of those
> types, so it wouldn’t really help.
>
>
>
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